The invention relates to a particle beam apparatus for tilted observation of a specimen which is capable of producing magnified images of the specimen under test with high accuracy.
Three-dimensional optical inspection and measurement of microscopic structures requires tilting of the beam axis with respect to the specimen surface. Known microscopes either use specimen tilt stages or mechanical tilting of the optical column. However, these methods have the following severe drawbacks: low throughput, large masses have to be accelerated and decelerated, mechanical drift, particle generation.
Furthermore, beam tilt is known from stereoscopic imaging techniques and convergent beam illumination techniques. However, since optical performance of a standard particle optical lens deteriorates quickly with increasing beam tilt angle, stereo microscopy is only feasible at relatively low magnification.
The object of the invention, therefore, is to provide a particle beam apparatus for tilted observation of a specimen which is capable of producing high magnification images of the specimen under tilted observation with optical resolution comparable to axial performance.
According to the invention, this object is achieved by the features of the present invention in that a multipole correction means is provided for correcting the lens aberrations occurring due to off-axial intersection of the lens by the tilted particle beam.
Further embodiments of the invention are also disclosed which further improve the performance of the particle beam apparatus.
FIG. 2 shows the influence of the spherical aberration in the particle beam apparatus occurring due to off-axial intersection of the lens by the tilted particle beam. A particle beam 1 is generated in a source 2 and is tilted by the deflection means 3 before the tilted particle beam 1a off-axially intersects a lens 4 which focusses the tilted particle beam la onto a specimen 5.
The lens 4 is an electrostatic, magnetic or combined electrostatic-magnetic objective lens. The refractive power of such a lens is too large for an off-axial beam. The off-axial beam forms a large spot 1b on the specimen 5. Even if the refractive power of the lens is reduced, the beam cannot be focussed into a small spot (comparable in size to the actual beam diameter) due to the cubic dependency of spherical aberrations on the tilt angle xcex1.
FIG. 3 shows the influence of the chromatic aberration in the particle beam apparatus occurring due to off-axial intersection of the lens by the tilted particle beam 1a. The refractive power of the lens 4 depends on the energy of the off-axial beam. Only electrons with the mean beam energy Eo are correctly focussed into a small spot on the specimen. Electrons with a lesser energy EOxe2x88x92xcex94E are focussed above the specimen plane and form a large spot on the specimen. Electrons with a larger energy EO+xcex94E are focussed (virtually) below the specimen plane and also form a large spot on the specimen. As a result, an elongated aberration figure is formed on the specimen and the beam cannot be focussed into a small spot (comparable in size to the actual beam diameter).
In order to correct one or both lens aberrations described in FIGS. 2 and 3, the invention proposes multipole correction means for correcting the lens aberrations occurring due to the off-axial intersection of the lens by the tilted particle beam.
In one embodiment, the correction means comprise at least eight pole elements for correcting the spherical aberration, and in another embodiment, the correction means comprise at least four electrostatic and four magnetic pole elements for correcting the chromatic aberration. In a third embodiment, both correction means are combined to correct the spherical as well as the chromatic aberration.